As we saw last week, in "Of Miracles", David Hume defined a miracle, rightly in my view, as "a transgression of a law of nature by a particular volition of the deity, or the interposition of some invisible agent." Do we have any reasons to suppose that any such miracles have occurred?

Hume thinks not, and at the core of his argument is an ingenious and simple principle. When it comes to matters of fact, the only thing we can rely on is evidence, of our own senses and that of others. So in deciding whether a miracle has taken place, we do no more than what we'd do when any other factual claim was made: we weigh up the evidence and see how the scales tip. From this simple, general principle, Hume specifies how this applies to miracles:

That no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish.

It's obvious really. Extraordinary claims require extraordinary evidence. The unbelievable only has to be believed when denying it creates more intellectual problems than accepting it. And it quickly becomes clear that no testimony for a miracle has ever passed the test. "There is not to be found, in all history," wrote Hume, "any miracle attested by a sufficient number of men, of such unquestioned good-sense, education, and learning, as to secure us against all delusion in themselves; of such undoubted integrity, as to place them beyond all suspicion of any design to deceive others; of such credit and reputation in the eyes of mankind, as to have a great deal to lose in case of their being detected in any falsehood; and at the same time, attesting facts performed in such a public manner and so celebrated a part of the world, as to render the detection unavoidable."

In every case of an alleged miracle, it is always more likely that it has a rational explanation than not, even if we do not know what that rational explanation is. For instance, let's say you see someone "rise from the dead". Pretty impressive. But given that all experience tells us this is impossible, no matter how striking your experience, it will always be more likely that you were somehow tricked or deceived. After all, we know the brain plays all sorts of tricks on us, and others play tricks on our brains.

But hasn't Hume set the bar too high? Imagine what you saw was a real miracle. Isn't Hume saying that you'd be compelled to conclude it was no such thing, and hence be wrong? In short, yes.

In a world of miracles, none of which was so clearly miraculous that it would be irrational to claim there was a natural explanation, reason would lead us to the false conclusion that no miracles had ever occurred. We should be sanguine about this. Hume did not believe reason was infallible, but we make fewer mistakes with it than without it.

So what Hume's argument boils down to, then, is that we have never had any good reason to believe that a miracle has occurred, and nor are we likely to. But this is not the same as saying dogmatically that no miracle ever has occurred. This explains the final paragraph of the essay, in which Hume seems to simultaneously clobber Christianity over head and give it a reprieve.

The Christian religion not only was at first attended with miracles, but even at this day cannot be believed by any reasonable person without one. Mere reason is insufficient to convince us of its veracity.

The same could be said for many other religions.

Believers may agree that reason is indeed insufficient, which is why you need faith. Hume agrees, but in spelling out what this faith means, he may make life a little uncomfortable:

And whoever is moved by faith to assent to it, is conscious of a continued miracle in his own person, which subverts all the principles of his understanding, and gives him a determination to believe what is most contrary to custom and experience.

You can believe in miracles as a matter of faith, not reason. But you have to realise that faith does not simply plug a gap where reason fears to tread; it actively goes against all that reason tells us.

There are plenty of serious believers prepared to bite that bullet, like Kierkegaard. But many others like to have their faith and keep all their reason too. Just as some like to make their faith more palatable to reason by falsely believing that miracles need not defy the laws of nature, so some hold that faith merely supplements reason and never actually supplants it. The miraculous prose of Hume shows that both convenient beliefs are hopeless illusions.

Read more blogs from Julian's series on Hume on the 'How to believe' page